Drilling tool

ABSTRACT

A drilling tool of the present invention includes: a casing pipe; a ring bit having a larger diameter and disposed coaxially on a tip side of the casing pipe; and a pilot bit inserted into the ring bit through the inside of the casing pipe. An outer peripheral part at a tip of the pilot bit is provided with a bit head which is extended when rotated in a tool rotation direction during drilling. The ring bit is provided with a part to be engaged that is engaged with the extended bit head in the tool rotation direction, and a first abutting part capable of abutting against the extended bit head in a direction of an axis.

TECHNICAL FIELD

The present invention relates to a so-called double pipe type drillingtool that performs drilling using a ring bit disposed on a tip side of acasing pipe, and a pilot bit inserted into the casing pipe.

Priority is claimed on Japanese Patent Application No. 2014-167602,filed Aug. 20, 2014, the content of which is incorporated herein byreference.

BACKGROUND ART

As such a double pipe type drilling tool, PTL 1 suggests a drilling toolin which a ring bit is rotatably inserted into a tip part of a casingpipe with their inner and outer periphery surfaces being made to faceeach other; an inner bit is attached to a tip of a transmission memberinserted into the casing pipe; striking force and impelling force aretransmitted to the casing pipe and rotating force in addition theretoare transmitted to the ring bit via this inner bit so as to performdrilling; and after a borehole having a predetermined depth is formed,the ring bit be removed from the casing pipe and left in the borehole.

PTL 2 describes a so-called under-reaming bit in which an extendable bitis attached to an outer periphery of a tip part of a shank devicerotated around an axis; a borehole having a predetermined internaldiameter is formed while the extendable bit is positioned in a extendingstate and protrudes to a tip of a casing pipe during drilling; and afterthe end of the drilling, the extendable bit is shrunk and then isrecovered with the shank device through the inside of the casing pipe.

CITATION LIST Patent Literature

[PTL 1] Japanese Patent No. 4887857

[PTL 2] Japanese Patent No. 4501407

SUMMARY OF INVENTION Technical Problem

In recent years, in specific drilling works using such drilling tools,there have been increasing numbers of cases in which a borehole isformed with a greater internal diameter than an internal diametersufficient enough to insert the casing pipe itself For example, in acase where a steel-pipe diaphragm wall is buried, a casing pipe in whicha joint is provided at an outer peripheral part of a steel pipe isinserted into a borehole. Further, in a case where the casing pipe isconnected by a coupling having a greater external diameter than theexternal diameter of the steel pipe, this coupling is inserted into theborehole, and therefore, a borehole with an internal diameter for whichthe external diameter to the joint or the coupling is taken intoconsideration must be formed. Moreover, also in a case where theperiphery of the steel pipe is cemented at a water well or the like forwater stoppage, and a borehole with a greater internal diameter than theexternal diameter of the steel pipe is required.

In a case where such a borehole with a greater internal diameter isformed by the double pipe type drilling tool described in PTL 1, theexternal diameter of the ring bit is increased. However, as describedabove, the ring bit is rotatably inserted with the outer peripheralsurface of the posterior end part thereof being made to face the innerperipheral surface of the casing pipe. Therefore, the internal diameterof the ring bit does not change and the size in the radial direction ofthe ring bit becomes larger. Since this ring bit is finally left in theborehole without being recovered, an increase in construction cost iscaused. Further, since the rotating force is transmitted to the ring bitvia the inner bit inserted into the ring bit through the inside of thecasing pipe, sufficient rotating force cannot be transmitted if theexternal diameter of the ring bit becomes larger, and also there is aconcern that drilling performance may degrade.

In a case where a borehole with a greater internal diameter is formedusing the under-reaming bit described in PTL 2, the radius of theextendable bit from the axis in the extending state is increased.However, there is also a limitation to enlarge the extendable bit inorder to shrink the extendable bit and recover it with the shank deviceafter the end of the drilling. Moreover, in forming a borehole having agreater internal diameter, load becomes larger. As a result, there isalso a concern that damage may occur on a shaft that rotatably supportsthe extendable bit. Further, there is also a limitation to the number ofdrilling tips disposed on the extendable bit, and drilling performancedegrades as the diameter of a borehole becomes larger.

The present invention has been made in view of such a background, andthe objective thereof is to provide a borehole that can preventdegradation of drilling performance, occurrence of damage, and anincrease in construction cost, in a case where a borehole with a greaterinternal diameter than the external diameter of a casing pipe isdrilled.

Solution to Problem

In order to solve the above problems and achieve the objective, thepresent invention provides a drilling tool including: a cylindricalcasing pipe centered on an axis; an annular ring bit that is coaxiallydisposed on a tip side of the casing pipe and has a greater externaldiameter than the casing pipe; and a pilot bit that is inserted into aninner peripheral part of the ring bit through the inside of the casingpipe. The pilot bit is rotatable around the axis and a bit head isprovided at an outer peripheral part of a tip of the pilot bit. The bithead is rotatable around a centerline eccentric from the axis, and isconfigured that when the pilot bit is rotated in a tool rotationdirection during drilling, the bit head is extended such that a radiusof the bit head from the axis is enlarged and thereby is supported bythe pilot bit. The ring bit is provided with: a part to be engaged thatis configured to be engaged with the extended bit head in the toolrotation direction during drilling; and a first abutting part capable ofabutting against the tip side of the extended bit head in a direction ofthe axis.

In this drilling tool, the bit head provided at the outer peripheralpart of the tip of the pilot bit is extended during drilling, and thefirst abutting part provided at the ring bit abuts against the tip sideof the extended bit head in the direction of the axis. Thus, the ringbit can be prevented from coming off to the tip side thereof. Also,since the part to be engaged of this ring bit is engaged with theextended bit head in the tool rotation direction during drilling,rotating force can be transmitted from the pilot bit via the bit head tothe ring bit.

Therefore, even if the external diameter of the ring bit is made to begreater than the external diameter of the casing pipe, via the extendedbit head of which the radius from the axis is enlarged, sufficientrotating force can be transmitted to the ring bit, and drillingperformance can be guaranteed. Additionally, when the part to be engagedof the ring bit is engaged with the extended bit head in the toolrotation direction during drilling, in this way, the need for making anouter peripheral surface of a posterior end part of the ring bit face aninner peripheral surface of the casing pipe to allow the posterior endpart to be rotatably inserted into the casing pipe is also eliminated.Therefore, the internal diameter of the ring bit can be increased, thatis, a required material can be reduced by making the volume of the ringbit small. Thus, even in a case where after the end of drilling, the bithead is rotated in a direction opposite to the direction during drillingand the ring bit is left in a borehole, construction cost can beprevented from increasing.

In contrast, in the pilot bit, even if the radius of the extended bithead from the axis is not made as large as the radius of the borehole, aborehole with a large internal diameter can be formed by the ring bit,and damage or the like to the bit head can be prevented without exertingan excessive load. Additionally, the number of the drilling tip disposedon the annular ring bit that drills the outer peripheral side of theborehole can be relatively freely set, and it is also possible toprevent degradation of drilling performance resulting from shortage ofchips.

Here, when a recessed portion that is recessed to an outer peripheralside is formed at the inner peripheral part of the ring bit, and therecessed portion serves as the part to be engaged, the volume of thering bit can be further reduced, and construction cost can be furtherreduced. In addition, in this case, a tip surface of the ring bitadjacent to the recessed portion may serve as the first abutting part,and a bottom surface that faces the tip side in the direction of theaxis may be formed in the recessed portion so as to serve as the firstabutting part.

In addition, in building the casing pipe in a borehole with the strikingforce and the impelling force to the tip side in the direction of theaxis to be applied to the pilot bit, as in the drilling tools describedin PTLs 1 and 2, a smaller-diameter part of which an internal diameteris one step smaller may be formed at an inner peripheral part of a tipof the casing pipe, and a second abutting part like a stepped partcapable of abutting against the smaller-diameter part from a posteriorend side in the direction of the axis may be formed at an outerperipheral part of a posterior end of the pilot bit so as to transmitthe striking force and the impelling force. In this case, by making theinternal diameter of the ring bit equal to or greater than the internaldiameter of the smaller-diameter part, the volume of the ring bit can bemade small as described above, and construction cost can be reliablyreduced.

In transmitting the striking force and the impelling force to the tipside in the direction of the axis to the ring bit, the striking forceand the impelling force may be directly transmitted to the ring bit fromthe pilot bit as in the drilling tool described in PTL 1. However, inthat case, when the smaller-diameter part is formed at the innerperipheral part of the tip of the casing pipe and is made to be capableof abutting against the stepped part of the pilot bit as describedabove, a smaller-diameter part of which the internal diameter is furthersmaller than the smaller-diameter part of the casing pipe must be formedat the ring bit to abut against the pilot bit, and consequently, thereis a concern that it becomes difficult to make the internal diameter ofthe ring bit small as described above to reduce construction cost.

Thus, particularly in such a case, by providing the bit head with athird abutting part which is capable of abutting against a surface ofthe ring bit that faces a posterior end side in the direction of theaxis in a state where the bit head is extended, it becomes unnecessaryto form a smaller-diameter part having a smaller internal diameter thanthe smaller-diameter part of the casing pipe in the ring bit,construction cost can be much more reliably reduced, and it is possibleto reliably transmit the striking force and the impelling force from thepilot bit to the tip side in the direction of the axis to the ring bitvia the third abutting part of the bit head.

Moreover, in this case, by making the greatest radius of the thirdabutting part of the extended bit head from the axis greater than aradius of an outer peripheral part of the tip of the casing pipe fromthe axis, the striking force and the impelling force can be transmittedto the outer peripheral side of the ring bit where drilling isperformed. In a case where a borehole with a greater internal diameterthan the external diameter of the casing pipe is formed, it is possibleto perform even more efficient drilling, the thickness of the ring bitin the direction of the axis can also be made small, and much moreconstruction cost reduction can be achieved.

Advantageous Effects of Invention

As described above, according to the present invention, even in a casewhere a borehole with a greater internal diameter than the externaldiameter of the casing pipe is formed, it is possible to transmitsufficient rotating force to the ring bit and to perform efficientdrilling, without causing degradation of drilling performance, anincrease in construction cost, or damage to the tool.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view showing a state where a bit head isextended in one embodiment of the present invention.

FIG. 2 is an enlarged front view when the embodiment shown in FIG 1 in astate where the bit head is retracted is seen from a tip side in adirection of an axis (illustration of a casing pipe and a casing top isomitted).

FIG. 3 is an enlarged front view when the embodiment shown in FIG. 1 ina state where the bit head is extended is seen from the tip side in thedirection of the axis (illustration of the casing pipe and the casingtop is omitted).

FIG. 4 is an enlarged front view when the ring bit of the embodimentshown in FIG. 1 is seen from the tip side in the direction of the axis.

FIG. 5 is Z-Z sectional view in FIG. 4.

DESCRIPTION OF EMBODIMENTS

FIGS. 1 to 5 show one embodiment of a drilling tool of the presentinvention. In the present embodiment, a casing pipe 1 is formed in acylindrical shape centered on an axis O using metallic materials, suchas a steel material, and a casing top 1A formed in a multi-stagecylindrical shape using metallic materials, such as a steel material, isattached to a tip part (left side in FIG. 1) of the casing pipe 1.

In the casing top 1A, the internal diameter thereof is a constantinternal diameter that is one step smaller than the internal diameter ofthe casing pipe 1, the external diameter of a tip part is the samediameter as the casing pipe 1, and the external diameter of a posteriorend part is of a size such that the posterior end part can be insertedand fitted into the casing pipe 1. The posterior end part of the casingtop 1A is inserted and fitted into the casing pipe 1 from a tip side ofthe casing pipe 1 and then jointed by welding or the like, whereby thecasing top 1A is coaxially integrated with the casing pipe 1.

By attaching the casing top 1A in this way, a smaller-diameter part 1Bof which the internal diameter is one step smaller is formed at an innerperipheral part of the tip of the casing pipe 1. In addition, aposterior end surface of the smaller-diameter part 1B is formed in theshape of a concave conical surface which is centered on the axis O andtilts toward an inner peripheral side so as to become slightly closer tothe tip side.

A pilot bit 2 is inserted into the casing pipe 1 from a posterior endside thereof (right side in FIG. 1). The pilot bit 2 is made of metallicmaterials such as a steel material and has an outer shape that is alsoformed in a multi-stage columnar shape, and a posterior end part thereofis made into a smaller-diameter shank part 2A. Striking force directedto the tip side in the direction of the axis O is transmitted to thepilot bit 2 from a down-the-hole hammer H attached to the shank part 2A.

Additionally, a drill rod (not shown) is added if necessary and iscoupled to the posterior end side of the down-the-hole hammer H, and adrill rod disposed at the most posterior end is attached to a drillingdevice. The impelling force directed to the tip side in the direction ofthe axis O and the rotating force directed in a tool rotation directionT during drilling are transmitted to the pilot bit 2 from the drillingdevice via the drill rod and the down-the-hole hammer H. In addition,the casing pipe 1 is also added to the posterior end side if necessaryand is inserted into a borehole.

A stepped part of which the external diameter becomes a maximum isformed at an outer periphery of the pilot bit 2 at a position closer tothe tip side than the shank part 2A, and serves as a second abuttingpart 2B of the present embodiment. The external diameter of the secondabutting part 2B is slightly smaller than the internal diameter of thecasing pipe 1 and greater than the internal diameter of thesmaller-diameter part 1B formed by the casing top 1A. Moreover, a tipsurface of the second abutting part 2B is formed in the shape of aconvex conical surface which tilts toward the inner peripheral side soas to become slightly closer to the tip side, and the tilt angle thereofis equal to the tilt angle of the posterior end surface of thesmaller-diameter part 1B that forms the concave conical surface shape.

Therefore, the pilot bit 2 is coaxial with the casing pipe 1 and thecasing top 1A movable integrally with the casing pipe 1 and the casingtop 1A to the tip side in the direction of the axis O, and rotatablearound the axis O relative to the casing pipe 1 and the casing top 1A,in a place where the pilot bit 2 is inserted into the casing pipe 1 fromthe posterior end side thereof and the second abutting part 2B abutsagainst the smaller-diameter part 1B. Additionally, the externaldiameter of the pilot bit 2 at a position closer to the tip side thanthe second abutting part 2B is a constant external diameter slightlysmaller than the internal diameter of the smaller-diameter part 1Bformed by the casing top 1A, and thus a tip part of the pilot bit 2 isformed so as to protrude greatly from the tip of the casing top 1A in astate where the second abutting part 2B abuts against thesmaller-diameter part 1B.

A housing recess 3 is formed at an outer periphery of the tip part ofthe pilot bit 2 protruding from the tip of the casing top 1A so as to belocated closer to the tip side than the casing top 1A. The housingrecess 3 includes: a bottom surface 3A that is located closer to the tipside than the casing top 1A, faces the tip side, and is perpendicular tothe axis O; and a wall surface 3B that extends to tip side in parallelwith the axis O from an inner peripheral edge of the bottom surface 3Aand reaches a tip surface of the pilot bit 2. The housing recess 3 isformed so as to be open to an outer peripheral surface and a tip surfaceof the tip part of the pilot bit 2. In the present embodiment, aplurality of (three) such housing recesses 3 having the same form andthe same size are formed at equal intervals in a circumferentialdirection.

A wall surface 3B of each housing recess 3 includes: a first wall part 3a that is a plane facing an outer peripheral side of the pilot bit 2; asecond wall part 3 b that is a plane located on a side opposite to thefirst wall part 3 a in the tool rotation direction T and facing the toolrotation direction T; and a third wall part 3 c that is a plane locatedon the same side as the first wall part 3 a in the tool rotationdirection T and facing the side opposite to the tool rotation directionT. The second and third wall parts 3 b and 3 c are formed such that agap therebetween in the circumferential direction becomes greater towardthe outer peripheral side, and the second wall part 3 b out of thesewall parts extends toward the outer peripheral side so as to tilt to thetool rotation direction T.

Additionally, a fourth wall part 3 d and a fifth wall part 3 e areformed in the shape of a concave cylindrical surface centered on astraight line parallel to the axis O at a boundary part between thefirst and second wall parts 3 a and 3 b and a boundary part between thefirst and third wall parts 3 a and 3 c, respectively. The fourth wallpart 3 d connects with the first and second wall parts 3 a and 3 b, andthe fifth wall part 3 e connects with the first and third wall parts 3 aand 3 c. The radius of the concave cylindrical surface as the fourthwall part 3 d formed at the boundary part between the first and secondwall part 3 a and 3 b is greater than the radius of the concavecylindrical surface as the fifth wall part 3 e formed at the boundarypart between the first and third wall parts 3 a and 3 c.

Moreover, a discharge groove 2C for cuttings is formed which extendstoward the posterior end side in parallel with the axis O from the toolrotation direction T side of the bottom surface 3A of each housingrecess 3 and reaches an outer peripheral side of the shank part 2Abeyond the second abutting part 2B. Each discharge groove 2C forms asubstantially rectangular shape in a cross-section perpendicular to theaxis O and is open to an outer peripheral surface of the tip part of thepilot bit 2. A bottom surface of each discharge groove 2C which facesthe outer peripheral side of the pilot bit 2 forms a recessed curvedshape and is slightly swept toward the outer peripheral side in a placewhere the bottom surface reaches a posterior end of the second abuttingpart 2B. A portion where this bottom surface and the bottom surface 3Aof the housing recess 3 intersect each other is chamfered by an inclinedsurface 2D that intersects with both the bottom surfaces at an obtuseangle.

On the other hand, a fitting hole 3C that has a centerline C parallel tothe axis O and has a circular cross-sectional shape is formed on theside opposite to the bottom surface 3A of each housing recess 3 in thetool rotation direction T. The centerline C of the fitting hole 3Ccoincides with the centerline of the concave cylindrical surface as thefourth wall part 3 d formed at the boundary part between the first andsecond wall parts 3 a and 3 b, and is eccentric to the outer peripheralside of the axis O. Additionally, the internal diameter (radius) of thefitting hole 3C is approximately equal to or slightly smaller than theradius of the concave cylindrical surface as the fourth wall part 3 d.

A bit head 4 is attached to each of the housing recesses 3 of the pilotbit 2. In the bit head 4, a columnar shaft part 4A to be inserted andslidably fitted into the fitting hole 3C and a head main body 4Bprovided on the tip side of the shaft part 4A are integrally formedusing metallic materials, such as a steel material. The bit head 4 isattached so as to be rotatable around the centerline C. The bit head 4is positioned in a state where as shown in FIG. 2, the head main body 4Babuts against the first wall part 3 a and is housed within the housingrecess 3 and the radius thereof from the axis O is reduced, or ispositioned in a state where as shown in FIG. 3, the head main body 4Babuts against the second wall part 3 b and the radius thereof from theaxis O is enlarged. A posterior end surface of the head main body 4B isa plane perpendicular to the centerline C.

A cutout 4C is formed at an outer periphery of the shaft part 4A suchthat the cutout 4C forms a semi-oval shape as shown in FIG. 1 in across-section along the centerline C and extends to form a substantialL-shape as shown in FIGS. 2 and 3 in a section perpendicular to thecenterline C. A pin 5 is driven into the tip part of the pilot bit 2 ina tangential direction of the fitting hole 3C in the cross-sectionorthogonal to the axis O, at a position that faces the cutout 4C in thedirection of the axis O in a state where the shaft part 4A is insertedinto the fitting hole 3C, a posterior end surface of the shaft part 4Ais made to abut against the bottom surface of the fitting hole 3C, and aposterior end surface of the head main body 4B is made to abut againstthe bottom surface 3A of the housing recessed portion 3. A peripheralsurface of each pin 5 is exposed inside the fitting hole 3C and isengaged with the cutout 4C, and thereby the bit head 4 is prevented fromcoming off to the tip side while being made to be rotatable around thecenterline C.

Additionally, a first side surface 4 a, which is located on an extensionof an outer peripheral surface of the shaft part 4A among side surfacesof the head main body 4B, is formed in the shape of a convex cylindricalsurface centered on the centerline C which flushes with this outerperipheral surface of the shaft part 4A or has an external diameterslightly greater than that of the outer peripheral surface of the shaftpart 4A. The first side surface 4 a is made to be slidable on the fourthwall part 3 d of the wall surface 3B of the housing recess 3. Moreover,second and third side surfaces 4 b and 4 c that sandwich the first sidesurface 4 a therebetween are formed in a planar shape. As shown in FIG.2, in a state where the bit head 4 is retracted, the second side surface4 b out of these side surfaces is made to abut against the first wallpart 3 a of the wall surface 3B of the housing recess 3 while the thirdside surface 4 c faces the outer peripheral side of the pilot bit 2. Ina state where the bit head 4 is extended, the third side surface 4 c ismade to abut against the second wall part 3 b while the second sidesurface 4 b is directed to the tool rotation direction T.

Moreover, a fourth side surface 4 d, which is located between the secondand third side surfaces 4 b and 4 c on a side opposite to the first sidesurface 4 a, is formed so as to protrude to the outer periphery of thepilot bit 2 and be located on a cylindrical surface centered on the axisO, as shown in FIG. 3 in a state where the bit head 4 is extended. Inaddition, an intersecting ridgeline part between the fourth side surface4 d and the third side surface 4 c is formed so as to be chamfered by acylindrical surface that has a diameter slightly smaller than theexternal diameter of the tip part of the pilot bit 2 and is centered onthe axis O in a state where the bit head 4 is retracted as shown in FIG.2. Accordingly, the head main body 4B retracted and housed in thehousing recess 3 is located inside a cylindrical surface of the outerperipheral surface of the tip part of the pilot bit 2.

Additionally, the fourth side surface 4 d is formed in a multi-stage(three-stage in the present embodiment) shape that becomes concave andconvex to the inner and outer peripheral sides with respect to the axisO toward the direction of the centerline C as shown in FIG. 1. A stageat the most posterior end among these stages is made to protrude to theoutermost peripheral side from the axis O, and a portion in which thestage at the most posterior end is formed serves as a third abuttingpart 4D in the present embodiment. The third abutting part 4D has asurface thereof facing the tip side which is a flat surfaceperpendicular to the centerline C, and a greatest radius R of the thirdabutting part 4D from the axis O is greater than a radius r of the outerperipheral parts of the tips of the casing pipe 1 and the casing top 1Afrom the axis O in a state where the bit head 4 is extended as shown inFIG. 1.

In addition, an intersecting ridgeline part between the fourth sidesurface 4 d and the second side surface 4 b in which the third abuttingpart 4D is formed is chamfered in the shape of a convex cylindricalsurface with a radius approximately equal to a concave cylindricalsurface as the fifth wall part 3 e of the housing recess 3, and as shownin FIG. 2, is made to abut against the fifth wall part 3 e in a statewhere the bit head 4 is retracted. Additionally, a stage at a foremostend of the fourth side surface 4 d slightly tilts toward the posteriorend side so as to become closer to the inner peripheral side in a stagewhere the bit head 4 is extended.

Moreover, at a middle stage located between the stage at the foremostend and a stage at a most posterior end in which the third abutting part4D is formed, the fourth side surface 4 d is formed so as to extend inparallel with the axis O. Furthermore, an engaging part 4E is formed ata corner part where that the fourth side surface 4 d the second sidesurface 4 b intersect with each other in this middle stage, such thatthe engaging part 4E cuts out the corner part in a substantial L-shapein a section orthogonal to the centerline C.

The engaging part 4E has a first wall surface 4 e facing the outerperipheral side and a second wall surface 4 f facing the tool rotationdirection T, in a state where the bit head 4 is extended, a bottomsurface 4 g that is made to be flush with a surface of the thirdabutting part 4D which faces the tip side, and a ceiling surface 4 hthat faces the bottom surface 4 g in parallel therewith and faces theposterior end side.

Similarly, in a state where the bit head 4 is extended, the first wallsurface 4 e is located on a cylindrical surface that has a slightlygreater external diameter than the second abutting part 2B of the pilotbit 2 and is centered on the axis O, and the second wall surface 4 f isformed such that the second wall surface 4 f extends toward the outerperipheral side so as to slightly tilt to the tool rotation direction T.

Moreover, an intersecting ridgeline part between a stage at a foremostend of the fourth side surface 4 d and a tip surface of the head mainbody 4B is formed as an inclined surface that extends toward thecenterline C so as to become closer the tip side in such a manner thesurface forms a truncated conical surface shape centered on the axis Oin a state where the bit head 4 is extended. Additionally, anintersecting ridgeline part between the tip surface and the outerperipheral surface in the pilot bit 2 is also formed as an inclinedsurface that similarly forms a truncated conical surface shape centeredon the axis O and tilts toward the inner peripheral side so as to becomecloser the tip side, except for the portion cutout by the housing recess3.

Furthermore, the tip surface of the pilot bit 2 and the tip surface ofthe head main body 4B of the bit head 4 except the portions made to havethese inclined surfaces are respectively flat surfaces perpendicular tothe axis O and the centerline C.

Additionally, the length of the head main body 4B in the direction ofthe centerline C is equal to the depth from the bottom surface 3A of thehousing recess 3 to the tip surface of the pilot bit 2. Therefore, thetip surfaces of the pilot bit 2 and the head main body 4B become flushwith each other in a state where the bit head 4 is housed in the housingrecess 3.

The tip surfaces and the respective inclined surfaces of the pilot bit 2and the head main body 4B of the bit head 4 are provided with aplurality (large number) of drilling tips 6 made of cemented carbide orthe like which is harder than a steel material or the like that formsthe pilot bit 2 and the bit head 4. Each of the drilling tips 6 is onein which, for example, hemispherical head part protruding from the tipsurfaces and the inclined surfaces and columnar trunk part which areintegrally formed, and is fixed by press-fitting, hot-shrink fitting,cold-shrink fitting, or brazing the trunk parts into each of circularholes formed perpendicularly to the tip surfaces and the inclinedsurfaces.

Moreover, an annular ring bit 7 is disposed coaxially with the axis O onthe tip side of the casing pipe 1. The ring bit 7 is also formed in anannular plate shape using metallic materials, such as a steel material,and a tip surface and a posterior end surface thereof that face thedirection of the axis O are perpendicular to the axis O. However, theintersecting ridgeline part between the tip surface and the outerperipheral surface is made to be a truncated cone-shaped inclinedsurface centered on the axis O. The drilling tips 6 made of hardmaterials, such as cemented carbide, are also provided on the inclinedsurface and the outer peripheral part of the tip surface so as toprotrude perpendicularly thereto, similar to the pilot bit 2 and the bithead 4.

Additionally, the external diameter of the ring bit 7 is greater thanthe external diameter of the casing pipe 1 and the casing top 1A andgreater than the external diameter of the extended bit head 4. Further,the internal diameter of the ring bit 7 is slightly greater than theexternal diameter of the second abutting part 2B of the pilot bit 2 andtherefore greater than the internal diameter of the smaller-diameterpart 1B formed within the casing pipe 1 by the casing top 1A. Theinternal diameter of the ring bit 7 is smaller than the externaldiameter of the extended bit head 4 and is of such a size that the firstwall surface 4 e of the engaging part 4E is fittable. Additionally, thethickness of the ring bit 7 in the direction of the axis O is smallerthan the width between the external and internal diameters of the ringbit 7, and is slightly smaller than a gap between the bottom surface 4 gand the ceiling surface 4 h of the engaging part 4E.

Moreover, three recessed portions that are recessed to the outerperipheral side and are of the same number as that of the bit heads 4are formed at equal intervals in the circumferential direction at theinner peripheral part of the ring bit 7, and as shown in FIG. 3, each ofthe recessed portions serves as a part 7A to be engaged that is engagedwith the engaging part 4E of each bit head 4 in the tool rotationdirection T during drilling.

The part 7A to be engaged includes a first wall surface 7 a that recedesby one step from the inner peripheral part of the ring bit 7 to theouter peripheral side and then faces the inner peripheral side, a secondwall surface 7 b facing the side opposite to the tool rotation directionT and a third wall surface 7 c facing the tool rotation direction Twhich extend from the first wall surface 7 a to the inner peripheralpart. In the present embodiment, the part 7A to be engaged is formed soas to pass through the ring bit 7 in the direction of the axis O.

The first wall surface 7 a among these surfaces is located on thecylindrical surface centered on the axis O. The radius of the first wallsurface 7 a from the axis O is slightly greater than the radius of thestage at the foremost end and the middle stage from the axis O in thefourth side surface 4 d of the extended bit head 4 which faces the outerperipheral side and is smaller than the radius R of the third abuttingpart 4D. Further, the circumferential length of the first wall surface 7a is slightly greater than a length except the engaging part 4E in thecircumferential length of the middle stage of the fourth side surface 4d.

Additionally, the second and third wall surfaces 7 b and 7 c extendtoward the outer peripheral side so as to tilt to the tool rotationdirection T, and an angle that the second wall surface 7 b out of thesewall surfaces makes with respect to a radial direction with respect tothe axis O is equal to an angle that the second wall surface 4 f in theengaging part 4E of the extended bit head 4 makes with the radialdirection with respect to the axis O. Further, as shown in FIG. 3, thethird wall surface 7 c is formed in a concave cylindrical surface shapecentered on the centerline C of the fitting hole 3C in the housingrecess 3 of the pilot bit 2, in a state where the part 7A to be engagedis engaged with the engaging part 4E.

In addition, in the pilot bit 2, a bottomed supply hole 8 is drilledfrom a posterior end of the shank part 2A along the axis O to thevicinity of a central part of the housing recess 3 in the direction ofthe axis O so as to be capable of supplying compressed air therethroughfrom the down-the-hole hammer H side. First to third blow holes 8A to8C, which are three for each and have a smaller diameter than the supplyhole 8, branch obliquely from the supply hole 8 and extend toward theouter peripheral side so as to tilt to the tip side.

Each first blow hole 8A is open to the tip side of the second abuttingpart 2B in the outer peripheral surface of the tip part of the pilot bit2. A fourth blow hole 8D with a much smaller diameter branches from eachfirst blow hole 8A in parallel with the axis O and is open to the centerof the bottom surface of the fitting hole 3C. Further, each second blowhole 8B branches from the supply hole 8 at a position closer to the tipside than each first blow hole 8A, and is open substantiallyperpendicularly to the inclined surface 2D between the bottom surface ofeach discharge groove 2C for cuttings and the bottom surface 3A of eachhousing recess 3. Moreover, each third blow hole 8C has a greaterdiameter than the first and second blow holes 8A and 8B, branches at thetip of the supply hole 8, and is open to the fifth wall part 3 e side ofthe first wall part 3 a of the housing recess 3.

In this drilling tool, the pilot bit 2 is inserted from the posteriorend side of the casing pipe 1 in a state where the bit head 4 isretracted and the head main body 4B is housed in the housing recess 3,and is positioned in the direction of the axis O in a place where thesecond abutting part 2B abuts against the posterior end surface of thecasing top 1A. Next, with the head main body 4B being housed, as shownin FIG. 2, a circumferential position of the part 7A to be engaged isaligned with the housing recess 3, and the ring bit 7 is inserted intothe tip part of the pilot bit 2 from the tip side and is disposed at theposition of the engaging part 4E of the head main body 4B in thedirection of the axis O.

When, from this state, the ring bit 7 is relatively rotated to the sideopposite to the tool rotation direction T during drilling while the bithead 4 is being extended, as shown in FIG. 3, the second wall surface 4f in the engaging part 4E of the extended bit head 4 comes in closecontact with and abuts against the second wall surface 7 b in the part7A to engaged of the ring bit 7 and thereby is engaged with the part 7Ato be engaged, and the third side surface 4 c in the head main body 4Babuts against the second wall part 3 b of the housing recess 3 and issupported by the housing recess 3. Thereby, the ring bit 7 becomesintegrally rotatable with respect to the pilot bit 2 and the bit head 4in the tool rotation direction T.

Additionally, in the direction of the axis O, as shown in FIGS. 1 and 3,when the portion of the part 7A to be engaged on the tool rotationdirection T side in the tip surface of the ring bit 7 faces the ceilingsurface 4 h of the engaging part 4E with a slight gap therefrom and isabuttable against the ceiling surface 4 h, the ring bit 7 is preventedfrom coming off to the tip side. That is, in the present embodiment, theportion of the part 7A to be engaged on the tool rotation direction Tside in the tip surface of the ring bit 7 serves as a first abuttingpart 7B that is abuttable against the extended bit head 4, on the tipside in the direction of the axis O. Moreover, the bottom surface 4 g ofthe engaging part 4E and the surface of the third abutting part 4D flushwith the bottom surface 4 g which faces the tip side abut against theposterior end surface of the ring bit 7, and support the ring bit 7 onthe tip side thereof. Thereby, the casing pipe 1 and the ring bit 7 aremade to be movable to the tip side in the direction of the axis Ointegrally with the pilot bit 2 and the bit head 4.

Therefore, when, from this state, the striking force directed to the tipside in the direction of the axis O is transmitted to the pilot bit 2and the bit head 4 and to the ring bit 7 via the third abutting part 4Dby the down-the-hole hammer H and the impelling force, and the rotatingforce directed in the tool rotation direction T are transmitted from thedrilling device, drilling work is performed by the drilling tips 6provided on the pilot bit 2, the bit head 4, and the tip surface of thering bit 7, and the casing pipe 1 is inserted into the formed borehole.In addition, compressed air is blown off from the supply hole 8 via thefirst to fourth blow holes 8A to 8D during drilling. Thereby, cuttingsgenerated by the drilling tip 6 are discharged through the inside of thecasing pipe 1 from the discharge groove 2C, and biting of the cuttingsinto the fitting hole 3C or the smaller-diameter part 1B is prevented.

After the borehole is formed up to a predetermined depth in this way, inthe drilling tool of the above configuration, the pilot bit 2 is rotatedto the side opposite to the tool rotation direction T during drilling bythe drilling device. Then, the head main body 4B of the bit head 4 isguided by friction with the borehole and by the third wall surface 7 cof the part 7A to be engaged, and thereby the bit head 4 is retracted asshown in FIG. 2. Thus, the pilot bit 2 and the bit heads 4 can berecovered with the ring bit 7 being left in the borehole by pulling outthe pilot bit 2 together with the down-the-hole hammer H as it is fromthe casing pipe 1.

In this way, according to the drilling tool of the above configuration,the rotating force in the tool rotation direction T is transmitted fromthe head main body 4B of the extended bit head 4 to the part 7A to beengaged of the ring bit 7. Thus, the rotating force can be efficientlytransmitted at a position farther from the axis O which becomes therotation center of the pilot bit 2 and the bit head 4. Accordingly, evenin a case where a borehole with a greater internal diameter than theexternal diameter of the casing pipe 1 is formed, sufficient rotatingforce can be transmitted to the ring bit 7, and drilling performance canbe guaranteed.

Moreover, in the present embodiment, the pilot bit 2 and the bit head 4protrude by one step to the tip side of the ring bit 7 as shown inFIG. 1. Therefore, the drilling tips 6 on the ring bit 7 drill an outerperipheral part of the borehole of which an inner peripheral part isdrilled by the drilling tips 6 on the pilot bit 2 and the bit head 4,and becomes apt to be crushed. For this reason, the load to the ring bit7 can be reduced, and more efficient drilling can be performed. Here thetip surfaces of the pilot bit 2 and the bit head 4 may be made to beflush with the tip surface of a ring bit 7, and the tip surface of thering bit 7 may protrude from the tip surfaces of the pilot bit 2 and thebit head 4.

Additionally, since the outer peripheral side of the borehole is drilledby the ring bit 7, in the pilot bit 2 and the bit head 4, it is notnecessary to make the radius of the extended head main body 4B from theaxis O as large as the internal diameter of the borehole. For thisreason, a burden to the shaft part 4A of the bit head 4 or the like canbe reduced, and damage can be prevented. Moreover, since the ring bit 7forms an annular shape, the number and positions of the drilling tips 6can be relatively freely set, for example, like the drilling tips 6being disposed in ranges other than a range in the circumferentialdirection where the extended bit head 4 as shown in FIG. 3 is located.Therefore, degradation of the drilling performance resulting frompartial insufficiency of the drilling tips 6 can also be prevented.

In the ring bit 7, the part 7A to be engaged is engaged with theextended bit head 4 as described above, while the ring bit 7 issupported by the bit head 4 so as to be rotatable integrally therewithin the tool rotation direction T and the rotating force is transmittedto the ring bit 7. Thus, it becomes unnecessary to support the ring bit7 with the casing pipe 1, and the internal diameter of the ring bit 7can be increased. For this reason, required materials such as a steelmaterial can be reduced by making the volume of the ring bit 7 small,and even in a case where the ring bit 7 is left in a borehole after theend of drilling, an increase in construction cost can be suppressed.

Additionally, as described above, the inner peripheral part of theborehole is drilled by the drilling tips 6 on the pilot bit 2 and thebit head 4. Thus, in the present embodiment, it is not necessary toprovide the ring bit 7 with the drilling tip 6, as shown in FIGS. 1 and3, in a range where the drilling tips are provided on the tip surface ofthe head main body 4B of the extended bit head 4 in the radial directionfrom the axis O. For this reason, it is possible to avoid providing moredrilling tips 6 made of expensive cemented carbide than needed in thering bit 7 left in the borehole, and cost reduction can be achieved.

Moreover, in the present embodiment, a recessed portion that is recessedtoward the outer peripheral side is formed in the inner peripheral partof the ring bit 7, and serves as the part 7A to be engaged. In thisregard, for example, it is also possible to form a protrusion on the tipsurface of the ring bit 7 as a part to be engaged to engage the headmain body 4B of the extended bit head 4 with this protrusion in the toolrotation direction T. However, in that case, there is a concern that theload resulting from the rotating force may be concentrated on theprotrusion to cause damage, and the volume of the ring bit 7 may alsobecome as great as the protrusion and material cost may increase. Incontrast, in the present embodiment, it is possible to receive therotating force with a main body itself of the annular ring bit 7, andthe volume and cost of the ring bit 7 can be further reduced.

In addition, in the present embodiment, the portion adjacent to the part7A to be engaged at the tool rotation direction T side thereof which isformed as the recessed portion recessed from the inner peripheral parttoward the outer peripheral side in this way in the tip surface of thering bit 7 faces the ceiling surface 4 h of the engaging part 4E, andserves as the first abutting part 7B abuttable against the ceilingsurface 4 h of the engaging part 4E on the tip side in the direction ofthe axis O. For example, even if the ring bit 7 collides against theceiling surface 4 h with the striking force transmitted via the bit head4 from the pilot bit 2, a shock can be received over the overallthickness of the ring bit 7, and occurrence of damage or the like can beprevented. However, the bottom surface that faces the tip side may beformed in this recessed portion so as to face the ceiling surface 4 h,and may be used as the first abutting part 7B.

Also in the present embodiment, in inserting the casing pipe 1 into aborehole with the striking force and the impelling force to the tip sideto be applied to the pilot bit 2, similar to the drilling toolsdescribed in PTLs 1 and 2, the casing top 1A is attached to the tip partof the casing pipe 1 to form the smaller-diameter part 1B, and thesecond abutting part 2B of the pilot bit 2 is made to abut against thesmaller-diameter part 1B so as to transmit the striking force and theimpelling force. However, in the present embodiment, the internaldiameter of the ring bit 7 is increased with respect to the internaldiameter of this smaller-diameter part 1B. Thus, as described above,construction cost can be reliably reduced compared to the drilling tooldescribed in PTL 1 in which the internal diameter of the ring bit has tobe made smaller. In addition, the internal diameter of the ring bit 7may be equal to the smaller-diameter part 1B.

Furthermore, in the present embodiment, the striking force and theimpelling force are transmitted by providing the casing pipe 1 with thesmaller-diameter part 1B in this way. In contrast, in transmitting thestriking force and the impelling force from the pilot bit 2 to the ringbit 7, the striking force and the impelling force are not directlytransmitted from the pilot bit 2 unlike the drilling tool described inPTL 1, but the bit head 4 is provided with the third abutting part 4Dthat is abuttable against the posterior end surface of the ring bit 7 inan extended state so as to transmit the striking force and the impellingforce from the third abutting part 4D. For this reason, in a case wherethe smaller-diameter part 1B is provided as described above, it isunnecessary to make the internal diameter of the ring bit 7 stillsmaller, and it is possible to reduce construction cost even morereliably.

Moreover, in the present embodiment, in a case where the striking forceand the impelling force are transmitted from the third abutting part 4Dprovided in the bit head 4 in this way to the ring bit 7, the greatestradius R of the third abutting part 4D of the extended bit head 4 fromthe axis O is greater than the radius r of the outer peripheral part ofthe tip of the casing pipe 1 from the axis O, that is, the radius of thecasing top 1A. For this reason, the striking force and the impellingforce can be more reliably transmitted to the ring bit 7 on the outerperipheral side where drilling is performed, and even in a case where aborehole with a greater internal diameter than the external diameter ofthe casing pipe 1 as in the present embodiment is formed, it is possibleto perform drilling much more efficiently.

Then, in a case where the striking force and impelling force aretransmitted from the third abutting part 4D that is enlarged and has thelarger radius R than the radius r of the outer peripheral part of thetip of the casing pipe 1 to the ring bit 7 in this way, it is possibleto reliably form a borehole with a large internal diameter withoutimpairing the strength or rigidity of the ring bit 7 even if thethickness of the ring bit 7 in the direction of the axis O is smallerthan the width between the external and internal diameters of the ringbit 7 like, for example, the present embodiment. Therefore, according tothe present embodiment, the volume of the ring bit 7 can be furtherreduced, and a much greater reduction of construction cost can beachieved.

INDUSTRIAL APPLICABILITY

As described above, according to the drilling tool of the presentinvention, even in a case where a borehole with a greater internaldiameter than the external diameter of the casing pipe is formed, it ispossible to transmit sufficient rotating force, striking force, andimpelling force to the ring bit and to perform efficient drilling,without causing degradation of drilling performance, an increase inconstruction cost, or damage to the tool. Therefore, the presentinvention can be industrially applied.

REFERENCE SIGNS LIST

1: CASING PIPE

1A: CASING TOP

1B: SMALLER-DIAMETER PART

2: PILOT BIT

2B: SECOND ABUTTING PART

2C: DISCHARGE GROOVE

3: HOUSING RECESS

3C: FITTING HOLE

4: BIT HEAD

4A: SHAFT PART

4B: HEAD MAIN BODY

4D: THIRD ABUTTING PART

4E: ENGAGING PART

5: PIN

6: DRILLING TIP

7: RING BIT

7A: PART TO BE ENGAGED

7B: FIRST ABUTTING PART

8: SUPPLY HOLE

O: AXIS OF CASING PIPE 1

T: TOOL ROTATION DIRECTION DURING DRILLING

C: CENTERLINE OF FITTING HOLE 3C

H: DOWN-THE-HOLE HAMMER

R: GREATEST RADIUS OF THIRD ABUTTING PART 4D OF EXTENDED BIT HEAD 4 FROMAXIS O

r: RADIUS OF OUTER PERIPHERAL PART OF TIP OF CASING PIPE 1 FROM AXIS 0

1. A drilling tool, comprising: a cylindrical casing pipe centered on anaxis; an annular ring bit that is coaxially disposed on a tip side ofthe casing pipe and has a greater external diameter than the casingpipe; and a pilot bit that is inserted into an inner peripheral part ofthe ring bit through an inside of the casing pipe, wherein the pilot bitis rotatable around the axis and a bit head is provided at an outerperipheral part of a tip of the pilot bit, wherein the bit head isrotatable around a centerline eccentric from the axis, and is configuredthat when the pilot bit is rotated in a tool rotation direction duringdrilling, the bit head is extended such that a radius of the bit headfrom the axis is enlarged and thereby is supported by the pilot bit, andwherein the ring bit is provided with: a part to be engaged that isconfigured to be engaged with the extended bit head in the tool rotationdirection during drilling; and a first abutting part capable of abuttingagainst the tip side of the extended bit head in a direction of theaxis.
 2. The drilling tool according to claim 1, wherein a recessedportion that is recessed to an outer peripheral side is formed at theinner peripheral part of the ring bit, and the recessed portion servesas the part to be engaged.
 3. The drilling tool according to claim 1,wherein a smaller-diameter part of which an internal diameter is onestep smaller is formed at an inner peripheral part of a tip of thecasing pipe; a second abutting part capable of abutting against thesmaller-diameter part from a posterior end side in the direction of theaxis is formed at an outer peripheral part of a posterior end of thepilot bit; and an internal diameter of the ring bit is equal to orgreater than the internal diameter of the smaller-diameter part.
 4. Thedrilling tool according to claim 1, wherein the bit head is providedwith a third abutting part which is capable of abutting against asurface of the ring bit that faces a posterior end side in the directionof the axis in a state where the bit head is extended.
 5. The drillingtool according to claim 4, wherein the greatest radius of the thirdabutting part of the extended bit head from the axis is greater than aradius of an outer peripheral part of the tip of the casing pipe fromthe axis.
 6. The drilling tool according to claim 2, wherein asmaller-diameter part of which an internal diameter is one step smalleris formed at an inner peripheral part of a tip of the casing pipe; asecond abutting part capable of abutting against the smaller-diameterpart from a posterior end side in the direction of the axis is formed atan outer peripheral part of a posterior end of the pilot bit; and aninternal diameter of the ring bit is equal to or greater than theinternal diameter of the smaller-diameter part.
 7. The drilling toolaccording to claim 2, wherein the bit head is provided with a thirdabutting part which is capable of abutting against a surface of the ringbit that faces a posterior end side in the direction of the axis in astate where the bit head is extended.
 8. The drilling tool according toclaim 3, wherein the bit head is provided with a third abutting partwhich is capable of abutting against a surface of the ring bit thatfaces a posterior end side in the direction of the axis in a state wherethe bit head is extended.
 9. The drilling tool according to claim 6,wherein the bit head is provided with a third abutting part which iscapable of abutting against a surface of the ring bit that faces aposterior end side in the direction of the axis in a state where the bithead is extended.
 10. The drilling tool according to claim 7, whereinthe greatest radius of the third abutting part of the extended bit headfrom the axis is greater than a radius of an outer peripheral part ofthe tip of the casing pipe from the axis.
 11. The drilling toolaccording to claim 8, wherein the greatest radius of the third abuttingpart of the extended bit head from the axis is greater than a radius ofan outer peripheral part of the tip of the casing pipe from the axis.12. The drilling tool according to claim 9, wherein the greatest radiusof the third abutting part of the extended bit head from the axis isgreater than a radius of an outer peripheral part of the tip of thecasing pipe from the axis.